Screw centrifuge

ABSTRACT

A screw centrifuge comprising inner and outer centrifuge rotors rotating at differential rotational speeds with a screw secured to the outer surface of the inner rotor for conveying separated solid matter. Means for supplying raw material to the separating chamber between the rotors, comprises an axially extending supply duct communicating with radial bushings secured in the inner rotor via a dismountable insert having corresponding, radially orientated discharge apertures.

United States Patent [72] Inventor Torben Boss Nielsen [56] ReferencesCited 1 N ggfi 'g Denmark UNITED STATES PATENTS Q f Dec24 '19683,187,997 6/1965 Gooch 233/7 [45] Patented 9 1971 3,279,687 10/1966Amero 233/7 [73] Assignee separamr AIS 3,428,246 2/1969 Finkelston 233/7Soborg, Denmark Primary Examiner-Robert W. Jenkins [32] Priority Jan.10, 1968 Att0rney-Stevens, Davis, Miller & Mosher [33] Denmark [31]74/68 [54] SCREW CENTRIFUGE ABSTRACT: A screw centrifuge comprisinginner and outer centrifuge rotors rotating at differential rotationalspeeds with a screw secured to the outer surface of the inner rotor forconveying separated solid matter. Means for supplying raw material tothe separating chamber between the rotors, comprises an axiallyextending supply duct communicating with radial bushings secured in theinner rotor via a dismountable insert having corresponding, radiallyorientated discharge apertures.

SCREW CENUGE This invention relates to a screw centrifuge having aseparating chamber defined between two rotors rotating at differentnumbers of revolutions, a conveyor screw located in the separatingchamber an secured to the internal rotor and a supply duct in theinternal rotor for supplying raw material to the separating chamberbetween the two rotors through one or more apertures in the wall of saidinternal rotor.

Such centrifuges, which are also called decanters, are employed, interalia, for a rough or preliminary separation of sludge such a sewersludge and refuse from destruction plants, into a partially dried phaseand a partially cleansed liquid phase, which, if desired, can both betreated further in other kinds of apparatus.

In the use of the centrifuge considerable wear may occur on the edges ofthe discharge aperture or apertures in the internal rotor, since thesolid matter frequently contains highly abrasive constituents,especially sand. This does not apply to sewer sludge only, but also todead animals, fish etc. With known centrifuges it is difficult to effectrepairs on the internal rotor, which repairs normally comprise buildingup, such as by welding, fresh material to replace the worn off material,because the apertures situated at the bottom of the screws helix areonly accessible with difficulty. Consequently, the operation istroublesome and time-consuming and, as a rule, it is impossible to carryit out in situ, because after having built up the worn parts by welding,it is often necessary to carry out a balancing of the rotor because ofits rather high rotational speed during operation of the centrifuge. L

It is a primary object of the invention to provide a centrifuge of thekind referred to in which the above mentionedrepair of the internalcentrifuge rotor can be carried out more readily than in the previouslyknown centrifuge and without requiring skilled labor and specializedequipment normally not available to the user of the centrifuge.

According to the present invention there is provided a screw centrifugecomprising a hollow internal rotor and an external rotor surroundingsaid internal rotor and defining therewith a separating chamber, meansfor rotatably supporting said rotors and means for rotating each of saidrotors at mutually different numbers of revolutions per minute, aconveyor screw secured to the outer side of, said internal rotor, meansfor supplying a raw material to be separated into said separatingchamber, said supply means comprising a supply duct extending axiallywithin said internal rotor and having spaced inlet and outlet ends, saidoutlet end being formed by an insert detachably secured within saidinternal rotor and having an axially facing inlet aperture and at leastone radially facing discharge aperture, at least one bushing detachablysecured in the wall of the internal rotor and having an inner end facelocated opposite a surface surrounding said discharge aperture of saidinsert, and means for separately discharging constituents of said rawmaterial from said separating chamber after separation thereof.

Hereby the repair of a worn rotor is considerably simplified, as it isonly necessary to replace the worn bushing or bushings and possibly thecentral insert with new parts which can be completely finished inadvance including the balancing and can be supplied as spare partstogetherwith the centrifuge. It is therefore readily possible to effectrepairs in situ, the centrifuge thus being out of operation after ashort time only. It is, furthermore, an advantage that the insert andthe bushings can, without unduly high expenses being incurred, bemanufactured from particularly wear-resistant material, e.g. steel thatis suitable for through-hardening, so that the service life is alreadyincreased by this means as compared to the known centrifuges.

The bushing may be provided with a collar which abuts against aninternal contact surface of the internal rotor, which collar providesfor a particularly simple assembly and securing of the bushing that canbe inserted from the interior of the internal rotor and subsequently bepressed against the contact surface.

To facilitate the mounting and removal of the insert which normally willbe situated approximately in the longitudinal center portion of theinternal rotor, the insert may be secured to an end flange on a shaftpart which is secured detachably to the internal rotor.

The insert may be cruciform or stellate and comprise a plurality ofradial discharge ducts which, at their inner ends are connected to aninlet duct extending axially in the insert and each discharge duct may,moreover, be connected with the inlet duct via a duct extendingobliquely within the insert. The oblique ducts mentioned counteract theaccumulation of sludge in the raw material in front of a transverse wallwhich, in the insert, delimits the radial discharge ducts, so that morequick discharge of the material to the separating chamber is obtained.

In the following the invention is described in greater detail whilereferring to the accompanying drawings, in which:

FIG. 1 shows a longitudinal section through a part of a screw centrifugeembodying the present invention;

FIG. 2 is a longitudinal section on a larger scale through the insertshown in FIG. 1 in the centrifuges internal rotor, which section istaken along lines II-II in FIG. 3;

FIG. 3 is a section along lines III-III in FIG. 2; and

FIG. 4 is a section along lines IV-lVin FIG. 2 also showing the fourdischarge bushings and an adjoining part of the internal rotor withouthelix winding or convolution.

The screw centrifuge illustrated in FIG. 1 comprises an outer orexternal rotor l which, in a manner not shown in detail, is journaledrotatably in the centrifuges frame and is coupled to a drivingmechanism. An :internal rotor 2 coaxial with rotor 1 is journaledrotatably in relation to the outer rotor 1 which, in FIG. 1 isillustrated by two diagrammatically shown bearings 3' and 4. The drivingmechanism, not shown, is arranged to cause rotors l and 2 to rotate atslightly differing numbers of revolutions per minute, whereby a screw 5fixed to the outside of the internal rotor 2 acts as conveyor of solidmatter which, in the annular separating chamber 6 delimited betweenrotors 1 and 2, is separated from a raw material supplied.

The raw material is supplied through a pipe 7 which, at its one end isfixed in the internal rotor 2 coaxial with same and which opens into aninsert 8 fixed in rotor 2, whereby the insert acts as distributorsupplying four radially mounted insert bushings 9 that have dischargeducts 10 opening into the separating chamber 6.

As a consequence of the rotation of the rotors l and 2, a centrifugingof the supplied raw material takes place, which thus is separated into asolid and a liquid phase. The rpm. and the direction of rotation of thetworotors 1 and 2 are chosen in such a manner that screw 5 transportsthe separated solid matter towards the right-hand side in FIG. 1 to anoutlet 11 at the right-hand end of rotor l. The liquid phase runs offthrough an overflow, not shown, at the left-hand end of rotor 1.

The insert 8, as shown in greater detail 181 FIGS. 2-4, has an axiallyextending hub 12,- the machined inner side of which encloses the end ofpipe 7, in the periphery of which there is provided an O-ring l3, videFIG. I, for sealing the connection. Internally of the hub 12, the insert8 has a transverse wall 14 with a central aperture 15 in line with thebore in the hub 12. The aperture 15 and the bore of hub 12 cooperate todefine an axial inlet duct in the insert. At a distance from thetransverse wall 14 the insert ends in a closed, terminal wall 16. Fromthe space between the two walls 14 and 16 issue four radial dischargeducts 17, which are in line each with one of the ducts 10 in the fourbushings 9 referred to above, there are, moreover, in the wall 14, foursmaller obliquely extending ducts 18, which each connect one of theradial ducts 17 with the space between the transverse wall 14 and thehub 12 where, as shown in FIG. 2, pockets 30 are formed in which solidmatter from the raw material might splash back from the wall 14 andsettle, if the ducts 17 were :not provided.

As is best seen from FIGS. 2 and 3, the main part of insert 8 iscross-shaped with four arms 19, each of which contains a radial duct 17.The radially outer terminal surface 20 of each arm is flat and lies,when the insert is mounted in the centrifuge, with a small clearanceopposite the radially inner end face on a bushing 9, vide FIGS. 1 and 4.Each bushing 9 is provided with a collar 21 abutting on an internalshoulder surrounding the bore in the rotor 2, wherein the bushing issecured, preferably with a firm fit.

The insert 8 is secured to a flange 23 by means of four bolts 22 whichflange, in turn, is welded to a tubular shaft 24, vide FIG. 1. Theflange 23 has a recess for receiving and centering a machined projection24 on the terminal wall 16 of the insert. As is seen from FIG. I, theshaft 24 serves partly for'the journaling of the internal rotor 2, inthat the bearing 3 is mounted on the extreme end of the shaft. Thetubular shaft with the insert 8 secured thereto is fixed to the rotor 2by means of eight bolts 26, the heads of which abut against a flange 27welded to the shaft 24, and the bolts are screwed into tapped holes inthe rotor 2. As is seen from FIG. 4, the four bolts 22 lie in thecorners between the four arms 19 of the insert end their heads abutagainst a ring 28, vide FIG. 1 which, in turn, abuts against machinedsurfaces 29 on the four arms of the insert, vide FIGS. 2 and 3.

For replacing the insert and/or one or more of the bushings 9, bolts 26are first loosened, after which the shaft 24 together with the insert 8can be removed axially from the internal rotor 2. The four collarbushings 9 are free after that and can be removed through the interiorof the rotor 2. After the dismantling of the tubular shaft and theinsert, these can be separated from each other by loosening of thescrews 22. The assembly of the parts is effected in reversed sequence.The bushings 9 normally do not require to have extremely firm or closefit in rotor 2, since the centrifugal force occurring at the centrifuges operation will assist in keeping them in place.

Iclaim:

l. A screw centrifuge comprising a hollow internal rotor and an externalrotor surrounding said internal rotor and defining therewith aseparating chamber, means for rotatably supporting said rotors and meansfor rotating each of said rotors at mutually different numbers ofrevolutions per minute, a conveyor screw secured to the outer side ofsaid internal rotor, means for supplying a raw material to be separatedinto said separating chamber, said supply means comprising a supply ductextending axially within said internal rotor and having spaced inlet andoutlet ends, said outlet end being formed by an insert detachablysecured within said internal rotor and having an axially facing inletaperture and at least one radially facing discharge aperture, at leastone bushing detachably secured in the wall of the internal rotor andhaving an inner end face located opposite a surface surrounding saiddischarge aperture of said insert, and means for separately dischargingconstituents of said raw material from said separating chamber afterseparation thereof.

2. A centrifuge as claimed in claim 1, wherein the bushing is providedwith a collar abutting on an internal surface of said internal rotorwhen said bushing is mounted.

3. A centrifuge as claimed in claim 1 and further comprising a shaftdetachably secured to said internal rotor and provided with an endflange, the insert being secured to said end flange.

4. A centrifuge as claimed in claim 1, wherein the insert is cruciformor stellate and comprises a plurality of radially extending dischargeducts, the inner end of each such ducts communicating with an inlet ductextending axially of said insert, and a plurality of obliquely extendingducts each connecting a discharge duct with the inlet duct.

1. A screw centrifuge comprising a hollow internal rotor and an externalrotor surrounding said internal rotor and defining therewith aseparating chamber, means for rotatably supporting said rotors and meansfor rotating each of said rotors at mutually different numbers ofrevolutions per minute, a conveyor screw secured to the outer side ofsaid internal rotor, means for supplying a raw material to be separatedinto said separating chamber, said supply means comprising a supply ductextending axially within said internal rotor and having spaCed inlet andoutlet ends, said outlet end being formed by an insert detachablysecured within said internal rotor and having an axially facing inletaperture and at least one radially facing discharge aperture, at leastone bushing detachably secured in the wall of the internal rotor andhaving an inner end face located opposite a surface surrounding saiddischarge aperture of said insert, and means for separately dischargingconstituents of said raw material from said separating chamber afterseparation thereof.
 2. A centrifuge as claimed in claim 1, wherein thebushing is provided with a collar abutting on an internal surface ofsaid internal rotor when said bushing is mounted.
 3. A centrifuge asclaimed in claim 1 and further comprising a shaft detachably secured tosaid internal rotor and provided with an end flange, the insert beingsecured to said end flange.
 4. A centrifuge as claimed in claim 1,wherein the insert is cruciform or stellate and comprises a plurality ofradially extending discharge ducts, the inner end of each such ductscommunicating with an inlet duct extending axially of said insert, and aplurality of obliquely extending ducts each connecting a discharge ductwith the inlet duct.